Spark Chasing (full harness rebuild)

[Gardy90]

Ill start off with, I have a 1952 M37 that I purchased on a whim 3 years ago. From the time I bought it the electrical system was in shambles. I had no working lights at all, and every wire was labeled differently than the schematic with painters' tape, and half-baked splices all over the place. Those first couple months that I owned it I tried and tried to get some working lights and get everything labeled but the amount of crusty insulation causing shorts all over I ended up giving up on that project. I settled for one taillight wired directly to the brake light pressure switch to give me a brake light to make it street legal during daylight hours. I drove it like this for almost 3 years, until last fall when I decided enough was enough.

My intent with this post is to share some of the tips/tricks and issues I ran into when rebuilding my wiring harness from the ground up. It may take me a few days in between posts to get the pictures and some videos sorted and get write ups done. When I first started looking into doing this I searched and searched and found very little in the way of specifics on the harness other than some grainy photos of a harness laid out, and the various schematics posted online.

This first round will be about the condition of my wiring as I found it. It had from what I can tell, the original harness with Douglas connectors. Turn signals were added later with the wires ran along the main harness and had Packard connectors. It had a solid-state flasher PN:11613663, and turn signal switch PN: M62069/1-2 (11613632-1) installed. From what I can gather with some googling, This turn signal switch is an early version with the indicator bulb at the end of the lever instead of under the lever as most replacements with the same part number have. The light switch was that standard 3 lever switch, and the ignition switch was a replacement that had screw down terminals. I tested both the light switch and the turn signal switch based on a diagram I found on the M37 facebook group. The turn signal switch I did not have a schematic for so I made my best guess based on the main full harness schematic. I will attach those to this first post, as well as the edit I made to highlight just the turn signal legs as well as the edits for how the solid-state flasher should be wired.

Prepping for this I spent the better part of a week and a full weekend tracing and verifying every wire I could before I started removing the harness. I started with tracing from the 3 lever switch using a pin saver on a long multimeter lead, having a multimeter with an audible indication of continuity is very helpful here unless you have someone else helping you. Thankfully the connections on the inside of the passenger side front fender were mostly intact giving me a decent starting point for the rear harness. Ill apologize I did not get many pictures of the rear harness removal, but ill add a few notes about it. It helps to have a tall jackstand to rest the fuel tank on. unless you cut the harness (I do not recommend since it will be harder to do the layout later) you will have to drop the rear of the fuel tank to get the connectors through the right rear of the frame/bed/bumper junction. After that the removal is straight forward, removing the retainer clips along the frame as you work your way to the front. It may be easier to pull it out from the rear after you clear that corner with the connectors. Taking off the floor pans on the passenger side floor may help with this as well.


Ill leave this as the first in this series, and try to get up one this weekend about rebuilding the rear harness.

 

[Gardy90]

I forgot to attach the light switch diagram. This is courtesy of Larry LaBahn on the M37 Facebook page.

 

[Gardy90]

Getting into the rear harness. I laid out plywood big enough to fit the entire harness to use as a layout board. The harness was stretched out along the board and i did a rough layout using screws to boarder the wire. This helped keep the old wires in place while i was unwrapping them as well as keep the new wires in place during layout and wrapping.


Every branch point had screws on both sides of the wire





After it was all laid out i began cutting the tape and loom off of the old wire. once all of the old loom and tape was removed you can carefully separate the wires from each other. There was a few wires/connections that were not accessible while it was installed into the truck so those were traced and tagged next.



One of the big differences i found between the shown schematic and this harness was how the splices in the rear harness are put together. There are two 2:1 douglas connectors in the middle of the rear of the harness. this is the "molded splice" shown on the schematic for wire #24 for the B/O tail lights this wire comes from the front of the harness, turns the right rear corner and goes into the first connector, coming out of that connector, one wire turns back to the right rear to the tail light connector, the 2nd wire coming out of this first connector goes into the 2nd connector that sits to the left side of the first connector. Of the two wires coming out of the left most 2:1 connector, one wire will go to the left hand tail light, and the other will go to the trailer connector. I dont have it pictured but the wire that splits off to the trailer connector has 1 more 2;1 connector mounted on top of the bracket splitting it into 2 different lines 24B and 490 for the trailer B/O Tail and Marker lights on the trailer.




Once all of the wires were labeled it was as simple as pulling one wire out at a time while replacing it with a new wire. This was done one at a time until the harness had all new wire laid out. The ends of wire were left about 6 inches longer on each end and will be trimmed when connectors are installed after verifying fit in the truck.



I then verified the wiring by setting up jumpers through the light switch, turn signal switch, and flasher, with some bare tail light internals hooked up to each side and light bulbs installed.



This whole process is straightforward. Install jumpers into the switches and harness as they appear on the schematic. The two batteries were jumped in as a ground wire ran to each corner of the rear, and to the light switch feed pin. I have a few videos i will post of the testing and a comprehensive overview of the whole completed harness. I did not get many if any at all pictures of wrapping the harness, but i went a little different route than factory. I first started by getting the wires laying straight with each other and not twisted best i could. I then put a small zip tie about every fist width to keep them in a tight bundle, i recommend starting at the right rear corner for this then working in each direction from there. After the whole harness was bundled tight i came back with 3M friction tape and taped around the branch points. I then followed this by using 3M non adhesive mastic tape (not sticky but almost permanently bonds to itself) wrapped 50% covering the previous turn in the wrap. This added a little more thickness than original but i think it was a good compromise as i wont have to add thicker loom in some of the rub areas and it should protect the wires for a good long time. Finally i followed this up with non adhesive vinyl tape for the finish wrap. This layer was wrapped 50% overlapping as well but started at the front of the harness, proceeded to the rear drivers side, and wrapped back all the way to the front. Final note on the wrapping, wrap your main branch points first before you begin wrapping the main runs, this will keep the junctions cleaner esp. when you are only doing one layer of the mastic tape.

Overall the rear harness is pretty simple and that's why i chose to start with it. If your harness is like mine where the turn signal wires were ran after the fact, try to keep those bundled together when you pull the harness out of the truck, I did not. this made for a little more work in running the wires but its easy to figure out if you run into that. The connections at the front are in the same place as the connections for the rest of the harness. the rear wires are ran to their respective sides, and each have a 2;1 connector inline to run a wire over to the trailer connector on the drivers side.

 

[Gardy90]